Circuit arrays may be used as multi-band front-end modules for mobile telephones. For example, a circuit array may be connected to the antenna of a mobile telephone at the antenna input. When the antenna comes into contact with an electrically-charged user, this can result in electrostatic discharge (ESD). Electrostatic discharges can create voltage peaks that are capable of destroying the circuit array. Thus, it is beneficial to equip circuit arrays of the type mentioned above with a device that protects against ESD.
Printed specification WO 00/57515 discloses circuit arrays that are equipped with a device for protecting against ESD. The protective device comprises an electric high-pass filter in which a capacitor is connected in series and an inductor is connected in parallel to the antenna input path.
A drawback of the known circuit array is that the ESD protective device only helps reduce the ESD impulse entering the circuit array directly through the antenna. In addition to the impulse entering the circuit array directly through the antenna, an electrostatic discharge can also generate a high voltage in the circuit array through ground coupling. This can occur, for example, because the control input normally used in a circuit is arranged either on the high voltage (high) or on a low voltage (low). In this case, the high voltage (high) is defined, for example, by the fact that it lies 2.3 V above the ground voltage of the circuit array. Because, in the case of a mobile telephone, as with many other devices based on signal transmission via antennas, the signal input runs from the antenna to the system's ground, an electrostatic discharge can also directly affect the ground voltage of the circuit array in a circuit array of the type mentioned above. As a result of the directly coupling of a control line to ground through the “high” condition, the voltage impulse resulting from an electrostatic discharge can not only affect the path, via the antenna, but also the circuit array, via the control line. The known circuit array is not protected against these effects.
In addition, the high-pass filter used in a known circuit array has the characteristic of being a very simply designed filter that allows all frequency components of a signal, beginning with a certain cutoff frequency, to pass through it virtually unimpeded. In general, however, only a very narrow frequency range is relevant to the further processing of the signal received by the antenna in the mobile telephone. For example, frequencies of between 1 and 2 GHz are used in mobile telephones based on the GSM, PCN, or PCS standard. All other frequencies received by the antenna tend to be interfering and, therefore, must be filtered out. Accordingly, at least one band-pass filter is needed to make the signal captured by the circuit array mentioned above capable of being processed for a mobile telephone. The high-pass filter arranged in the known circuit array can only cut off frequencies below a certain cutoff frequency. For this reason, at least one filter circuit must be connected downstream from this filter, in order to remove the frequency range relevant to the mobile telephone from the signals received by the antenna.
An advantage of the known circuit array is that the high-pass filter circuit used to protect against ESD has an insertion loss, causing the wanted signals to undergo a certain attenuation, although cutting of the transmitted frequency band does not take place. A disadvantage of the known circuit array is a generally high insertion loss.